1. Field of the Invention
The present invention relates to a method and apparatus for X-ray analysis using a wavelength-dispersive X-ray spectrometer (WDS) and, more particularly, to measurement and display of X-ray spectra.
2. Description of Related Art
Electron probe microanalyzers (EPMAs) and X-ray fluorescent analyzers (XRF) measure X-ray spectra and perform qualitative and quantitative analyses, using wavelength-dispersive X-ray spectrometers (WDS).
FIG. 4 is a diagram illustrating the principle of WDS equipped to an EPMA. This X-ray spectrometer has a curved X-ray analyzing crystal whose center C moves on a straight line that is tilted by a takeoff angle a from a point S from which X-rays are produced. At this time, the point S, center C, and the center D of a slit in the X-ray detector are kept on a Rowland circle having a constant radius of R. Furthermore, the distance SC is kept equal to the distance CD. The analyzing crystal C having a crystal lattice plane curved with a radius of curvature of 2R always faces the center O of the Rowland circle.
The distance SC is referred to as the spectral position L. Let θ be the angle of incidence of X-rays on the analyzing crystal. As can be seen from FIG. 4, the following relationship holds:L=2R×sin θ  (1)Meanwhile, regarding the diffraction conditions for the analyzing crystal, the following relationship holds:2d×sin θ=n×λ  (2)where λ is the wavelength of X-rays and d is the lattice interplanar spacing of the analyzing crystal. n is a diffraction order assuming a positive integer. Eqs. (1) and (2) lead toL=(2R/2d)×n×λ  (3)The relationship between the wavelength λ of X-rays and the spectral position L can be known from Eq. (3).
An X-ray spectrum having a horizontal axis on which wavelength λ (or any one of a corresponding energy value, spectral position L, the value of sin θ, and the value of 2θ) is plotted and a vertical axis on which X-ray intensity is plotted can be obtained by scanning across the spectral position L and, at the same time, measuring X-rays that are counted by an X-ray detector.
Characteristic X-rays produced from chemical elements constituting a substance have wavelengths intrinsic to the respective elements. The kinds of the elements contained in a sample to be analyzed can be known by knowing the wavelength λ of the characteristic X-rays (qualitative analysis). The concentrations of the contained elements can be known by knowing the intensities of the characteristic X-rays (qualitative analysis). For example, JP-A-2002-181745 sets forth a conventional technique for performing a simple quantitative analysis by collecting X-ray spectra by WDS, identifying chemical elements from the spectra, and making a comparison with the previously found X-ray intensity of a reference sample using the characteristic X-ray peaks of the identified elements.
Counting of X-rays involves a random process. When the average of collected counts is N, the variance based on statistical fluctuations also produces N counts. Therefore, in the conventional X-ray spectrum acquired under the condition where the count time per point is kept at a constant value, collected counts near peak tops have the greatest amount of variation (statistical fluctuations). Consequently, it is impossible to obtain accurate waveforms from short-time measurements where the count time per point is constant.
FIG. 5 is a graph showing the results of a simulation made to know whether the profile of a spectrum near peak tops having some height is affected by the presence or absence of variations in X-ray X counts. A spectrum (P) is obtained on the assumption that peaks having no variations are derived. In contrast, a spectrum (Q) is obtained based on X-ray counts in a case where X-ray counts collected at various spectral positions are subjected to variations due to a random process.
In order to obtain an accurate X-ray spectral waveform by minimizing variations in X-ray counts, one conventional technique consisting of increasing the constant count time per point corresponding to each spectral position is available. The whole X-ray spectrum has been measured for a long time. In another conventional method available, the whole X-ray spectrum is measured in a short time. Then, a region close to peaks is again measured for a long time.
However, these techniques fail to meet a demand for a technique capable of performing an analysis in a practically minimum time. Therefore, JP-A-51-25184 discloses a technique for lowering the scanning speed of a spectrometer only near existing peaks after detecting whether there are characteristic X-ray peaks. Furthermore, JP-A-1-312449 discloses a technique consisting of preparing a sample to be analyzed, previously setting a wavelength range for the sample, and making a long measurement of only the wavelength range in which characteristic X-ray peaks of chemical elements that might be contained in the sample appear.
In the technique of JP-A-51-25184, as the scanning speed of the spectrometer is lowered near peaks, the feed speed of a chart on which an X-ray spectrum is recorded is lowered synchronously. However, measurements of X-ray spectra using recent EPMA or XRF are performed by highly sophisticated digital control systems and so no chart is used. Furthermore, if X-ray intensities exceed a preset background level, the measure taken is only to switch the scan speed of the spectrometer to a lower speed. If peak heights vary variously, the lower speed used after the switching is kept constant irrespective of peak heights. Because characteristic X-ray peaks have heights that are different by plural orders of magnitude, it is impossible to set the scan speed according to the various peak heights.
In the technique of JP-A-1-312449, an X-ray spectrum is accepted as digital data under computer control. A wavelength range near characteristic X-ray peaks that are forecasted to appear is simply measured under constant conditions. It is impossible to set measurement conditions according to different peak heights.